I love myself some advanced stats and Batum is absolutely awesome offensively according to Synergy, although I was surprised when they rated him poorly on defense (although 82games says he's pretty darn good based on opponent's PER).
In any case, it's a matter of money. Batum has gotten better every year and I think many have high hopes for the guy - I think he could certainly work beautifully with Rondo. However, like, I said, it's all about the price.
Another issue is that, at least next year, he'll play the same position as Paul and Green (assuming we want him to come back). It seems a little strange to have all three on the roster, so something's going to have to give.